Sure, using "u" isn't that confusing, but the boundary has to be set somewhere, this was a friendly piece of advice, no-one has yet been shot for a "u"... (well, actually about 20 "u" prisoners get shot every day, but Garner hushes it up...)

If you dont tell someone for a small thing like that, they may think it's ok and go on to whole posts full of "OMG WTF u R such a g33k !"... Admittedly everyone wouldn't necessarily go that far, but it's worth telling people just in case. And also, as a community that focuses on written works, good writing is appreciated, although of course genuine mistakes due to dyslexia or english-as-an-Nth-language are fine...